Process for making dialkyl carbonates

ABSTRACT

A process for producing dialkyl carbonates, such as dimethyl carbonate, from the reaction of a primary alcohol with urea in the presence of a novel organotin catalyst complex with a high boiling electron donor compound acting as solvent which are (1) materials having the general formula RO[CH 2 (CH 2 ) k CH 2 O] m R, wherein each R is independently selected from C 1-12  alkyl, alkaryl or aralkyl moieties, k=0,1, 2 or 3 and m=1, 2, 3, 4 or 5 and (2)  bidentate ligand which form 1:1 bidentate and/or 1:2 monodentate adducts with R′ 2 SnX 2 (X═Cl, R′O, R′COO or R′COS), R′ 3 SnX, R′SnO, Ph 3-n R′SnX n  or Ph 4-n SnX n  (wherein R′=C q H 2q-1 n=0, 1 or 2 and q=2  1 to 12) and mixtures thereof, such as materials having the general formula RO[CH 2 ( CH   2 ) x   CH   2   O]   m   R, wherein each R is independently selected from C   1-12    alkyl, alkaryl or aralkyl moieties, k=0,1, 2 or 3 and m=1, 2, 3, 4, or 5.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for the production of dialkylcarbonates, particularly dimethyl carbonate wherein the reaction occurssimultaneously with separation of the reactants and the carbonateproducts. More particularly the invention relates to a process whereinmethanol is reacted with urea and/or alkyl carbamate in the presence ofa novel catalyst complex comprising a homogeneous organic tin compoundand an electron donor oxygen atom containing compound.

2. Related Art

Dialkyl carbonates are important commercial compounds, the mostimportant of which is dimethyl carbonate (DMC). Dimethyl carbonate isused as a methylating and carbonylating agent. It can also be used as asolvent to replace halogenated solvents such as chlorobenzene. Althoughthe current price of dimethyl carbonate is prohibitively expensive touse as fuel additive, it could be used as an oxygenate in reformulatedgasoline and an octane component. Dimethyl carbonate has a much higheroxygen content (53%) than MTBE (methyl tertiary butyl ether) or TAME(tertiary amyl methyl ether), and hence not nearly as much is needed tohave the same effect. It has a RON of 130 and is less volatile thaneither MTBE or TAME. It has a pleasant odor and, unlike ethers, isbiodegradable.

In older commercial processes dimethyl carbonate was produced frommethanol and phosgene. Because of the extreme toxicity and cost ofphosgene, there have been efforts to develop better, non-phosgene basedprocesses.

In one new commercial process, dimethyl carbonate is produced frommethanol, carbon monoxide, molecular oxygen and cuprous chloride viaoxidative carbonylation in a two step slurry process. Such a process isdisclosed in EP 0 460 735 A2. The major shortcomings of the process arethe low production rate, high cost for the separation of products andreactants, formation of by-products, high recycle requirements and theneed for corrosion resistant reactors and process lines.

Another new process is disclosed in EP 0 742 198 A2 and EP 0 505 374 B1wherein dimethyl carbonate is produced through formation of methylnitrite instead of the cupric methoxychloride noted above. Theby-products are nitrogen oxides, carbon dioxide, methylformate, etc.Dimethyl carbonate in the product stream from the reactor is separatedby solvent extractive distillation using dimethyl oxalate as the solventto break the azeotropic mixture. Although the chemistry looks simple andthe production rate is improved, the process is actually verycomplicated because of the separation of a number of the materials,balancing materials in various flow sections of the process, complicatedprocess control and dealing with the hazardous chemical, methyl nitrite.

In another commercial process dimethyl carbonate is produced frommethanol and carbon dioxide in a two step process. In the fist stepcyclic carbonates are produced by reacting epoxides with carbon dioxideas disclosed in U.S. Pat. Nos. 4,786,741; 4,851,555 and 4,400,559. Inthe second step dimethyl carbonate is produced along with glycol byexchange reaction of cyclic carbonates with methanol. See for example Y.Okada, et al “Dimethyl Carbonate Production for Fuel Additives”, ACS,Div. Fuel Chem., Preprint, 41(3), 868, 1996, and John F. Knifton, et al,“Ethylene Glycol-Dimethyl Carbonate Cogeneration”, Journal of MolecularChemistry, vol 67, pp 389-399, 1991. While the process has itsadvantages, the reaction rate of epoxides with carbon dioxide is slowand requires high pressure. In addition the exchange reaction of thecyclic carbonate with methanol is limited by equilibrium and methanoland dimethyl carbonate form an azeotrope making separation difficult.

It has been known that dialkyl carbonates can be prepared by reactingprimary aliphatic alcohols such as methanol with urea in the presence ofvarious heterogeneous and homogeneous catalysts such as dibutyltindimethoxide, tetraphenyltin, etc. See for example P. Ball et al,“Synthesis of Carbonates and Polycarbonates by Reaction of Urea withHydroxy Compounds”, C1Mol. Chem., vol 1, pp 95-108, 1984. Ammonia is aby-product and it may be recycled to urea as in the following react onsequence.

Carbamates are produced at a lower temperature followed by production ofdialkyl carbonates at higher temperature with ammonia being produced inboth steps.

As noted the above two reactions are reversible under reactionconditions. The order of catalytic activity of organotin compounds isR₄Sn<R₃SnX<<R₂SnX₂, wherein X═Cl, RO, RCOO, RCOS. A maximum reactionrate and minimum formation of by-products are reported for dialkyl tin(IV) compounds. For most catalysts (Lewis acids), higher catalystactivity is claimed if the reaction is carried out in the present of anappropriate cocatalyst (Lewis base). For example, the preferredcocatalyst for organic tin (IV) catalysts such as dibutyltindimethoxide, dibutyltin oxide, etc. are triphenylphosphine and4-dimethylaminopyridine. However, the thermal decomposition ofintermediate carbamates to isocyanic acid (HNCO) or isocyanuric acid((HNCO)₃) and alcohols is also facilitated by the organotin compoundssuch as dibutyltin dimethoxide or dibutyltin oxide employed in thesynthesis of aliphatic carbamates. WO 95/17369 discloses a process forproducing dialkyl carbonate such as dimethyl carbonate in two steps fromalcohols and urea, utilizing the chemistry and catalysts published by P.Ball et al. In the first step, alcohol is reacted with urea to producean alkyl carbamate. In the second step, dialkyl carbonate is produced byreacting further the alkyl carbamate with alcohol at temperatures higherthan the first step. The reactions are carried out by employing anautoclave reactor. However, when methanol is reacted with methylcarbamate or urea, N-alkyl by-products such as N-methyl methyl carbamate(N-MMC) and N-alkyl urea are also produced. The dialkyl carbonate ispresent in the reactor in an amount between 1 to 3 weight % based ontotal carbamate and alcohol content of the reactor solution.

SUMMARY OF THE INVENTION

Dialkyl carbonates are prepared by reacting alcohols with urea or alkylcarbamate or both in the presence of a organic Group IVA (IV) complexsuch as a dibutyltin dimethoxide complex wherein the reaction ispreferably carried out in the reboiler of a distillation still withconcurrent distillation of the dialkyl carbonate.

In a preferred embodiment the complexing agents are high boiling organicelectron donor compounds which have one, two, three, four or more oxygenatoms per molecule, preferably two or more oxygen atoms per moleculepreferably polyglycol ethers such as triglyme (triethylene glycoldimethyl ether), whose boiling point is preferably higher than eithermethanol or dimethyl carbonate and which serve as both cocatalysts andsolvent. Thus, the present invention provides an improved process byconcurrently distilling dialkyl carbonate away from the reactionconcurrently with the reaction and by preferably using specificcomplexing agents for the organotin catalyst.

The preferred electron donor oxygen atom containing compounds useful ascocatalyst and/or solvent comprises (1) materials having the generalformula RO[CH₂(CH₂)_(k)CH₂O]_(m)R, wherein each R is independentlyselected from C₁₋₁₂ alkyl, alkaryl or aralkyl moieties, k=0, 1, 2 or 3and m=1, 2, 3, 4 or 5 and (2) bidentate ligands which form 1:1 bidentateand/or 1:2 monodentate adducts with R′₂SnX₂ (X=Cl, R′O, R′COO or R′COS),R′₃SnX, R′SnO, Ph_(3-n)R′SnX_(n) or Ph_(4-n)SnX_(n) (whereinR′=C_(q)H_(2q-1) n=0, 1 or 2 and q=2 to 12) and mixtures thereof, suchas materials having the general formula RO[CH₂(CH ₂)_(k) CH ₂ O] _(m) R,wherein each R is independently selected from C ₁₋₁₂ alkyl, alkaryl oraralkyl moieties, k=0, 1, 2 or 3 and m=1,2, 3, 4 or 5 . In additionthese materials may be admixed with higher hydrocarbons, preferablyhaving 8 to 12 carbon atoms, such as dodecane and xylenes.

Examples of ligands which form 1:1 bidentate and/or 1:2 monodentateadducts with R′₂SnX₂ include diethylene glycol ether, 1,3-dimethoxypropane, 1,2-dimethoxypropane, dipropylene glycol dimethyl ether,1,4-dioxane, di-n-butyl ether and the like.

One advantage of producing dimethyl carbonate from urea and methanol isin the separation of dimethyl carbonate from the reaction mixture. Sincewater is not coproduced, the reaction mixture (the overhead product)does not form a ternary azeotrope and, hence the separation of theproduct dimethyl carbonate form the overhead mixture is easier than thecurrent commercial processes which have to deal with such a ternaryazeotrope.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an apparatus that can be employed to carry outthe present invention.

FIG. 2 is a plot of dimethyl carbonate in overhead versus hours onstream comparing methanol only with methanol+triglyme.

FIG. 3 is a plot of dimethyl carbonate in overhead versus hours onstream comparing methanol only with methanol+triglyme andmethanol+triglyme+DMAP at a 1.5 cc/min overhead rate.

FIG. 4 is a plot of methylamine in overhead versus hours on streamcomparing methanol only with methanol+triglyme andmethanol+triglyme+DMAP at a 1.5 cc/min overhead rate.

FIG. 5 is a plot of dimethyl carbonate in overhead versus hours onstream comparing methanol only with methanol+triglyme at a 2.7 cc/minoverhead rate.

FIG. 6 is a plot of dimethyl carbonate in overhead versus hours onstream for a single step process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The reaction is preferably carried out in the presence of a high boilingelectron donor oxygen containing solvent, which also serves as thecomplexing agent with the organotin compoun compound, by employing thereboiler of distillation still as the reactor. The reactor temperatureis controlled by changing the overhead pressure of the distillationcolumn. The use of the reboiler and distillation column allows effectiveremoval of the reaction products, dimethyl carbonate and ammonia, whilekeeping the homogeneous catalyst and solvent in the reactor. The columnmay be of any conventional form such as trays, packing, or combinationsthereof.

The novel organotin catalyst complex may be prepared by mixing organotincompound with high boiling electron donor oxygen containing compounds,such as ethers, usually at room temperature, in situ in the reactionzone, e.g. the reboiler at the initiation of the dialkyl carbonatereaction. When organotin halides, acetates or oxides are used as thecatalyst precursors, the complex formation may be carried out prior tothe initiation of the dialkyl carbonate, in order to remove the acid orwater, which is generated in the complexing reaction, although it is notnecessary or preferably to do so, since the acidic component and waterare easily remove overhead during the startup of the dialkyl carbonatereaction.

The reaction order of 2-methylhexyl carbamate in the presence of excess2-ethylhexyl alcohol has been proposed to be pseudo first order, or lessthan one. Therefore a lower methanol concentration relative to a givenconcentration of methyl carbamate is expected to be favorable for higherconversion rate of methyl carbamate. The use of both thereboiler-distillation column, and the high boiling oxygen atom(s)containing solvent such as diglyme (diethylene glycol dimethyl ether),triglyme (triethylene glycol dimethyl ether) or tetraglyme(tetraethylene glycol dimethyl ether), etc, allows carrying out thereaction under any desired pressure while maintaining any desiredconcentration of reactants (methanol, urea and carbamate) and product(dimethyl carbonate) in the reaction zone to obtain the best economicalresult.

In choosing the high boiling ethereal solvent the dialkyl carbonateproduced in the reaction is a consideration. For example triethylenedimethyl ether is preferred for the production of dimethyl carbonate,but it is not preferred for the production of diethyl carbonate, becausethe product is contaminated with methyl ethyl carbonate and the solventis slowly converted to triethylene diethyl ether. A preferable solventfor diethyl carbonate production would be tri- or tetraethylene glycoldiethyl ether.

In the present invention the desired ratio of the solvent to methanol inthe reaction medium is controlled by changing the ratio of methanol tohigh boiling electron solvent at a given concentration of carbamate or agiven combined concentration of urea and carbamate in the reboiler. Theuse of the high boiling electron donor solvent such as triglyme as acocatalyst as well as a part of the reaction medium overcomes theshortcomings of the earlier processes.

Despite the high yield or selectivity of carbonates claimed in WO95/17369 and P. Ball et al (C₁ Mol. Chem., 1, 95, 1984 and ACS, Div. ofPolymer Chemistry, Polymer Preprints, 25, 272, 1984 and C₁ Mol. Chem.,1984, Vol. 1, pp. 95-108) it is important to understand thedecomposition of urea and carbamate. Urea can decompose thermally orcatalytically to isocyanic acid and ammonium isocyanate or biuret(N₂NCONHCONH₂) under the reaction conditions employed to prepare dialkylcarbonates (D. J. Belson et al., Chemical Soc. Reviews, 11, 41-56,1982). Analysis of the overhead vent gas taken during the reactionindicates some carbon dioxide produced. Although P. Ball et al statedthat aliphatic carbamates can be distilled without decomposition,carbamates also can decompose thermally or catalytically to isocyanic orisocyanuric aid and alcohol (J. J. Godfrey, U.S. Pat. No. 3,314,754), orform allophanates (ROCONHCONH₂) H. W. Blohm and E. I. Becker, Chem.Rev., 1952, 51, 471). Ball et al stated that the thermal decompositionof the carbamates into isocyanic acid and alcohol competes with theformation of carbonate. However, the paper claims that thisdecomposition does not occur in the presence of suitable cocatalysts forsome catalysts. Triphenylphosphine and 4-dimethylaminopyridine are citedas good cocatalysts for organotin catalysts. Present EXAMPLES 4A and 4Bindicate that methyl carbamate decomposes thermally or catalytically inthe presence of organotin catalysts.

Dimethyl carbonate is a highly active compound so in order to improvethe selectivity to dimethyl carbonate, the concentration of dimethylcarbonate in the reboiler should be kept as low as possible. In thepresent invention a very low concentration of dimethyl carbonate isobtained by selecting the proper high boiling solvent and controllingthe overhead pressure which is a function of the ratio of methanol tohigh boiling electron donor solvent in the reboiler at a givenconcentration of methyl carbamate or given combination of methylcarbamate and urea. The use of the high boiling electron donor compoundsas both cocatalyst and solvent improves the rate of forming dialkylcarbonates (because of effective removal of both ammonia and dimethylcarbonate from the reaction zone) and, at the same time, prevents theformation of by-products such as N-alkyl alkyl carbamate, alkyl amine,and N-alkyl urea or decomposition of urea or carbamate at relativelyhigh concentration of dialkyl carbonate in both reactor and overheadproducts. High concentration of dialkyl carbonate in the overheadproduct reduces the cost of separating the dialkyl carbonate frommethanol for recycle.

Since the reaction can be carried out at lower pressures (less than 100psig) the new process has a number of advantages; lower cost for thematerial of construction, low catalyst inventory cost, easier removal ofthe ammonia and dimethyl carbonate products, and ease of control of theoptimum concentration of methanol in the reactor for the maximumdimethyl carbonate formation rate and selectively to dimethyl carbonate.

Flushing out the reactor (reboiler) with an inert gas such as nitrogenalthough not necessary, be included as as part of the startup. If inertgas is used to flush the reboiler the lower pressures preferably used inthe present reaction system allows for the use of a blower instead of acompressor for the inert gas.

The preferred range of reactor temperature is from 270 to 400° F.,preferably from 300-380° F. The preferred overhead pressure is withinthe range of 10-250 psig, more preferable between 20-200 psig and mostpreferably between 25-150 psig. The desirable weight ratio of highboiling electron donor solvent to methanol in the reactor is from100-0.01:1, preferably 5-0.1:1. The preferred concentration of organotincompounds in the reactor is from 0.5 to 40 wt. %, preferably from 2-30wt. % based on the total content in the reactor. The preferred overheadproduct rate is controlled to have from 4-35 percent by weight dimethylcarbonate, preferably form 5-25 wt. %. The preferred concentration ofmethyl carbamate or combined concentration of methyl carbamate and ureain the reactor is from 5-60 wt. %, preferably from 15-55 wt. % duringcontinuous operation.

For the continuous production of dimethyl carbonate, the urea solutionmay be directly pumped into the reactor or partially or completelyconverted to methyl carbamate prior to pumping into the reactor. Suchconversion could be accomplished in a feed preheater or in separatereactor. The solvent for the urea solution can be substantially puremethanol or very dilute dimethyl carbonate solution in methanol. Anexample of the dilute dimethyl carbonate solution (about 2% dimethylcarbonate in methanol) is the overhead recycle stream from a dimethylcarbonate recovery column. In one embodiment all or a portion of theurea solution may be fed to the distillation column instead of thereboiler to partially convert urea to methyl carbamate prior to enteringthe reboiler. In another embodiment material from the reboiler may beadded to the distillation column with the urea feed steam or at someother point along the column.

Referring now to FIG. 1 is a schematic representation of theexperimental apparatus used for the following examples is shown. The 350ml reboiler 10 of the still equipped with a stirring blade 12 was usedas the reactor. The ¾″ diameter distillation column 20 was packed with⅛″ ceramic saddles. Reactants, solvent and catalyst were charged to thereboiler 10 at ambient temperature (−75° F.). The reactions were carriedout by raising the reboiler temperature to a selected temperature bycontrolling the overhead pressure of the column. During the reaction thereactants were pumped into the reboiler continuously. The reactionproducts were removed from the reboiler as overhead product from thecolumn 20 via flow line 30 and condensed in condenser 40 where theammonia was removed as vapor via flow line 50 and product dimethylcarbonate removed via flow line 60. During the reaction, the liquidvolume in the reboiler was maintained at a preferred constant level bypumping in additional methanol, methanol-solvent mixture or thesolutions of urea or methyl carbamate in methanol or methanol-solventmixture through flow line 70. As an option a portion of the urea can befed directly to the distillation column via flow line 90 as shown. In asimilar manner catalyst complex from reboiler 10 may be fed directly tothe distillation column via flow line 100. Samples for analysis wereremoved from the reboiler via flow line 80. Samples from the overheads(total overheads) and bottoms were analyzed by gas chromatograph. Thereboiler temperature was controlled by controlling the overheadpressure. To raise the temperature the overhead pressure was raised.When the high boiling oxygen atom containing solvent was used, the novelorganotin complex catalyst was formed by mixing dibutyltin dimethoxideand the solvent such as triglyme together in the reboiler. The reactionsystem in the reactor may be characterized as homogeneous.

EXAMPLE 1A

The reboiler of the distillation still was charged with 96 g of urea,112 g of methanol, 113 g of triglyme (triethylene glycol dimethyl ether)and 25.5 g catalyst (dibutyltin dimethoxide) in the reboiler and thenraising the reboiler temperature to the desired temperature withstirring. During the heating and reaction, 5 weight percent triglymesolution in methanol was continuously pumped into the reboiler tomaintain constant liquid level in the reboiler. The reboilertemperatures during the reaction were maintained by controlling overheadpressure. When the reboiler temperature reached the desired temperature(320° F.), the withdrawal of the overhead liquid product was started atthe 1 cc/m rate through the line 60. At the beginning of the reaction,the reboiler temperature was maintained at 320° F. for an hour and thenat 355° F. until shut-down. The overhead pressures at 320° F. and 355°F. at the beginning were 100.6 and 106.5 psig, respectively. The columntemperatures were 325° F. at the bottom section of the column and 253°F. at the top section of column at 355° F. reboiler temperature at thebeginning of the reaction. The overhead pressure was lowered as theconversion of urea to methyl carbamate progressed. At the end (6 hourson stream) of the run, the overhead pressure was 68.1 psig. The columntemperature was 295° F. at the bottom section of column and 234° F. atthe top section of the column. The overhead liquid products werecomposed of methanol and dimethyl carbonate with small amount ofdissolved ammonia. No triglyme was observed in the overhead products.The change of the composition of the overhead liquid products during therun are illustrated in FIG. 2. While the bottom product sample taken atthe end of the 6 hour run contained 7.2% dimethyl carbonate and 22.6%methyl carbamate, the overhead product contained 16.0% dimethylcarbonate. The content of urea in the bottoms product sample was unknownbecause urea could not be analyzed by gas chromatography due todecomposition of urea.

EXAMPLE 1B

The reaction was carried out in the identical manner to EXAMPLE 1Awithout the cocatalyst. The reboiler was charged with 96 g of urea, 200g of methanol and 50.1 g of dimethyltin dimethoxide. The reboilertemperatures during the reaction were maintained by controlling overheadpressure. The flow rate of the overhead liquid product was 1 cc/min. Tomaintain a constant liquid level in the reboiler during the run, puremethanol was pumped into the reboiler. At the beginning of the reaction,the reboiler temperature was maintained at 320° F. for an hour and thenat 365° F. until shut-down. The overhead pressures at 320° F. and 365°F. at the beginning were 163 and 261 psig. The column temperature was342° F. at the bottom section of the column and 325° F. at the topsection of the column at the beginning of 365° F. reboiler temperature.The pressure was raised as the reaction progresses to maintain 365° F.reboiler temperature. At the end (7.1 hours on stream) of run, theoverhead pressure was 357 psig. The column temperature was 362° F. atthe bottom section of the column and 353° F. at the top section of thecolumn. The change of the composition of the overhead liquid productsduring the run are illustrated in FIG. 2. While the bottom productsample taken at the end of 7.1 hour run contained 7.8% dimethylcarbonate and 2.2% methyl carbamate, the overhead product contained only1.5 dimethyl carbonate. The content of urea in the bottoms productsample was unknown because urea could not be analyzed by gaschromatography due to decomposition of urea.

EXAMPLE 1A demonstrates how effectively the product carbonate is removedfrom the reaction zone in the present invention as shown in FIG. 2. Whenthe reaction is carried out according to the preferred embodiment of thepresent invention starting from urea, the product dimethyl carbonatecontent in the overhead increases rapidly as urea is converted to methylcarbamate which in turn is converted to dimethyl carbonate. Because ofrelatively low pressure of the still, the product dimethyl carbonate iseffectively removed as overhead product from the reboiler as indicatedby the concentration of dimethyl carbonate in the overhead and bottomproducts at the end of run; more dimethyl carbonate (16%) in theoverhead product than in the bottom product (7.2%). The sample takenfrom the reboiler contained 22.6% methyl carbamate. Urea is effectivelyconverted to dimethyl carbonate with little indication of decompositionof urea and methyl carbamate. When the reaction is carried out withouttriglyme as in Example 1B, less product dimethyl carbonate is removed asthe overhead product as shown in FIG. 2. Dimethyl carbonate isaccumulated in the reboiler by effective rectification of the columnunder higher pressure rather than removed as the overhead mixture. Theoverhead product at the end of run contains only 1.5% dimethyl carbonatewhich compares with 7.8% dimethyl carbonate in the bottom product.Undesired side reactions of product dimethyl carbonate and thedecomposition of urea and carbamate occur because of the accumulation ofDMC in the reboiler and absence of cocatalyst such that at the end ofrun, there are only 2.2% methyl carbamate and 7.8% dimethyl carbonate inthe reboiler.

EXAMPLE 2 CATALYST COMPLEX

When dibutyltin dimethoxide (liquid at room temperature) was mixedtogether with methanol, ethyl ether or toluene, dibutyltin dimethoxidecatalyst was completely soluble in these solvents and detectable by gaschromatography if analyzed the solutions by using TCD detector and DB-5gas chromatography column. Dibutyltin dimethoxide in methanolic solutionor toluene was detectable (5.38 minutes retention time) by gaschromatography. The analysis of the dibutyltin dimethoxide solution indiethyl ether indicated that the organotin compound in the solution wasno longer dibutyltin dimethoxide. The tin compound in the solution wasmuch heavier than dibutyltin dimethoxide so that the peak for the neworganotin complex compound in the ether solution has about 3 time longerretention time (15.68 min.). When dibutyltin dimethoxide was mixed withmixtures of triglyme and methanol or pure triglyme(CH₃OCH₂CH₂OCH₂CH₂OCH₂CH₂OCH₃) at room temperature, white fluffyprecipitate was formed which slowly saddled down at the bottom ofbottles. If the white precipitate suspended solution was filteredimmediately and the clear filtrate was analyzed with gas chromatographythe organotin compound was still detectable, although the concentrationwas lower than expected. With low concentration (10 weight %) of theorganotin catalyst in the mixed solution, the white precipitatedissolved completely to become clear solution by standing at roomtemperature overnight. If the white precipitate suspended solution waswarmed, the white precipitate immediately became soluble. When theseclear solutions were analyzed with gas chromatography, the dimethyltindimethoxide was no longer detectable.

EXAMPLE 2 demonstrates the formation of adduct complex compounds by thereaction of dibutyltin dimethoxide (Bu₂Sn(IV)(OCH₃)₂) with the electrondonor etherial compounds. In the present invention, the complexcompounds are employed as catalysts which are the adducts of theelectron donor oxygen atom(s) containing molecules to the organotincompounds. The catalytic complex compound was prepared by simply mixingdibutyltin dimethoxide and triglyme or other electron donor etherialcompounds together in the reboiler of the distillation still.

EXAMPLE 3A

The reboiler of the distillation still was charged with 125 g methylcarbamate, 100 g methanol, 100 g triglyme and 24.7 g dibutyltindimethoxide. The reboiler temperature was maintained at 355-363° F. bycontrolling the overhead pressure. The flow rate of the overhead liquidproduct was set at 1.5 cc/min. To maintain a constant liquid level inthe reboiler, a mixture of methanol and triglyme prepared by mixing 1650g methanol with 142.5 g triglyme was continuously pumped into thereboiler. The reaction was carried out for 6 hours each day for 2 days,for a total of 12 hours. After a 6 hour run, the unit was shut down. Onthe following day the unit was restarted. During the reaction theoverhead liquid products were collected into a reservoir. At the end ofthe run all the composite overhead liquid product in the reservoir andthe inventory materials in the reboiler and column were removed from thesystem and weighted and then analyzed. During the run the samples takenfrom the unit for analysis were weighted. The result of this experimentis listed in Table 1. The change in the compositions of dimethylcarbonate and methylamine in the overhead liquid products during the runare illustrated in FIG. 3 and 4 respectively. The overhead pressures at355° F. at the beginning and the end were 53.4 psig and 139 psig,respectively. The column temperatures at the bottom and top section ofthe column were 234° F. and 200° F. at the beginning, and 288° F. and277° F. at the end of 12 hours run. The analysis of the bottom productsample taken from the reboiler at the end of 12 hours run indicated 0.1%ammonia, 4.1% dimethyl carbonate, 0.3% N-MMC, 2.7% methyl carbamate,32.6% methanol, and 60.2 triglyme. The overhead product contained 6.9%dimethyl carbonate. The content of urea in the bottom product sample wasunknown, because urea could not be analyzed by gas chromatography due todecomposition of urea.

The EXAMPLE 3A demonstrates the superior yield and selectivity fordimethyl carbonate of the present invention compared with the prior art.The dimethyl carbonate content in the overhead liquid product of thepresent invention was at least 3 times higher than the dibutyltindimethoxide alone (Example 3B). Consequently the separation of dimethylcarbonate from the overhead product can be achieved at much lower costand much reduced amount of material recycle. When only methanol is usedas the solvent (Example 3B), the increase of the overhead liquid productrate slowly improves the selectivity of dimethyl carbonate and reducesthe formation of undesired by-products such as N-MMC (N-methyl methylcarbamate) and methylamine, however, the amount of the undesiredby-products are still about 10 times higher than the preferredembodiment of in the present invention. The increase of the overheadproduct rate with the cocatalyst has little effect on the dimethylcarbonate selectivity. The increase of the overhead rate simply dilutesthe concentration of dimethyl carbonate in the overhead product stream.The overhead products from the present invention generally contain nomethylamine or at most trace amounts.

EXAMPLE 3B

The reaction was carried out by utilizing the distillation still. Therun was carried out in the identical manner to the EXAMPLE 3A butwithout the cocatalyst. The reboiler was charged with 125 g methylcarbamate, 200 g methanol and 25.3 g dibutyltin dimethoxide. The flowrate of the overhead product was set at 1.5 cc/m. Methanol wascontinuously pumped into the reboiler to maintain a constant liquidlevel in the reboiler. The result is listed in Table 1. The change inthe compositions of dimethyl carbonate and methylamine in the overheadliquid products during the run are illustrated in FIG. 3 and 4,respectively. The overhead pressures at 355° F. at the beginning and theend were 268.4 and 374.4 psig respectively. The column temperatures atthe bottom and top section of the column were 332° F. and 321° F. at thebeginning, and 353° F. and 348° F. at the end of 12 hours run. Theanalysis of the bottom product sample taken from the reboiler at the endof 12 hours indicated trace ammonia, 6.9% dimethyl carbonate, 3.6%N-MMC, 2.1% methyl carbamate, 86.6 methanol, and 0.7% others. Theoverhead product contained 2.1% dimethyl carbonate and 2.5% methylamine.The content of urea in the bottom product sample was unknown becauseurea could not be analyzed by gas chromatography due to ureadecomposition.

EXAMPLE 3C

Triglyme was used as solvent to distill off the dimethyl carbonateproduct from the reaction zone, and the reaction was carried out byutilizing the distillation still. The experiment was carried out in theidentical manner to the EXAMPLE 3A. The reboiler was charged with 125 gmethyl carbamate, 100 g methanol, 21 g 4-dimethylaminopyridine (DMAP)cocatalyst, 79 g triglyme and 24.4 g dibutyltin dimethoxide. The run wascontinued for 12 hours without interruption. To maintain a constantliquid level in the reboiler, a mixture of methanol and triglymeprepared by mixing 1650 g methanol with 142.5 g triglyme wascontinuously pumped into the reboiler. The result is listed in theTable 1. The change in the compositions of dimethyl carbonate andmethylamine in the overhead liquid products during the run areillustrated in FIG. 3 and 4, respectively. The reboiler temperature wasmaintained at 344-357° F. by controlling the overhead pressure. Theoverhead pressures at 355° F. at the beginning and the end were 58.9 and81 psg, respectively. The column temperatures at the bottom and topsection of the column were 238° F. and 226° F. at the beginning and 256°F. and 245° F. at the end of 12 hours run respectively. The analysis ofthe bottom product sample taken from the reboiler at the end of 12 hoursrun indicated 0.2 dimethyl carbonate, 4.2% N-MMC, 0.5 methyl carbamate,18.5% methanol, 7.0% 4-dimethylaminopyridine, 69.3% triglyme and 0.3%others. The overhead product contained 1.4% dimethyl carbonate and 0.2methylamine. The concentration of urea in the bottom product sample wasunknown but expected to be very low.

TABLE 1 Example 3A 3B 3C Solvent 100 MeOH- 200 MeOH 100 MeOH- 100 TG 79TG-21 DMAP Reboiler 355 355 355 Temp, ° F. Ovhd P, psig initial 53.4268.4 58.9 final 139 374.4 81 Rate, cc/min 1.5 1.5 1.5 Mass Balance, %103.1 105.7 89.9 Mole Balance*, % 94.0 41.0 17.4 Apparant MC 95.9 93.298.9 conv., m % Apparant Selectivities*, m % DMC 90.2 31.8 9.7 N-MMC 0.68.9 6.8 DMC removed 99.2 23.2 13.6 as ovhd product, g *The urea contentin the reboiler was not included in the calculation. TG; triglyme DMAP;4-dimethylaminopyridine

In EXAMPLE 3C, 4-dimethylaminopyridine (DMAP) was used as cocatalyst asdisclosed in the Cl Mol. Chem., 1984, Vol 1, 95-108, Ball et al,“Synthesis of Carbonates and Polycarbonates by Reactions of Urea withHydroxy Compound”. In addition triglyme also was used to carry out thereaction under low pressure according to the present invention. SinceDMAP is much stronger Lewis base than triglyme the effect of triglyme onthe catalyst as cocatalyst is minimal. As shown in Table 1 and FIG. 3,the organotin complex compound catalyst, which is adduct complex(BU₂Sn[OCH₃]₂×xDMAP, wherein x=1 or 2 or both), has very poorselectivity for dimethyl carbonate. The analyses of the overhead vaporsamples taken during the run confirmed the decomposition of methylcarbamate, because the gas samples contained very large volumes ofcarbon dioxide. In EXAMPLE 3C the analyses of the overhead liquidproduct and the bottom product taken at the end of 12 hours indicatethat dimethyl carbonate was effectively removed from the reaction zonebecause the composition of dimethyl carbonate in the overhead productwas 7 times higher than that of the bottom product. The formation ofundesired by-product (N-MMC and methylamine) was much higher thanEXAMPLE 3A, but somewhat better than the EXAMPLE 3B, probably due toeffective removal of dimethyl carbonate and ammonia during the run.

EXAMPLE 4A COMPARISON

The thermal decomposition of methyl carbamate was carried out. A methylcarbamate (12.3 weight percent) solution was prepared by dissolving 35 gmethyl carbamate in 250 g triglyme in a 350 ml round bottom flask whichwas equipped with reflux column. The solution was refluxed with stirringat 338 to 356° F. After 12.45 hours reaction, the solution contained8.2% methyl carbamate and 0.9% methanol, indicating thermaldecomposition of methyl carbamate. The color of solution changed fromwater clear at the beginning to clear golden yellow solution, indicatingformation of heavier compounds, and very small amount of white solid wasdeposited at the bottom of condenser.

EXAMPLE 4B COMPARISON

The catalytic decomposition of methyl carbamate was carried out by usingthe same equipment set-up in the EXAMPLE 4A. A methyl carbamate (11.9weight percent) solution in triglyme was prepared by dissolving 35 gmethyl carbamate in 250 g triglyme and then adding 10 g dibutyltindimethoxide to the solution. After 4 hours reaction at 313 to 320° F.,the solution contained 7.6 weight percent methyl carbamate, 1.4 weightpercent methanol and only 0.8 weight percent dimethyl carbonate,indicating the faster decomposition rate catalyzed by dibutyltindimethoxide-triglyme complex. The color of the solution changed fromwater clear at the beginning to clear orange and some white solid wasdeposited at the bottom of condenser indicating again formation ofheavier materials which could not be detected with gas chromatography.After 12 hours reaction the solution contained only 2.7 weight percentmethyl carbamate, 1.2 weight percent methanol, and 2.7% dimethylcarbonate. Visual examination indicated that the amount of white solidmaterial deposited at the bottom of reflux column was increased slightlycompared with the EXAMPLE 4A.

EXAMPLE 5A

The experiment was carried out in the same manner in the EXAMPLE 3A. Thereboiler of the distillation still was charged with 125 g methylcarbamate, 100 g methanol, 100 g triglyme and 25.0 g dibutyltindimethoxide. The reboiler temperature was maintained at 352-356° F. bycontrolling the overhead pressure during the reaction. The flow rate ofthe overhead liquid product was set at 2.5 cc/min. A methyl carbamatesolution prepared by dissolving 120 g methyl carbamate in a mixedsolution of 1960 g methanol and 40 g triglyme was pumped into thereboiler to maintain the constant liquid level in the reboiler. Thereaction was terminated after 12 hours on stream. The result of thisexperiment is listed in Table 2. The overhead pressures at the beginningand the end were 39.4 psig and 123.9 psig, respectively. The columntemperatures at the bottom and top section of the column were 224 and211° F. at the beginning, and 283 and 272° F. at the end of 12 hoursrun. The analysis of the sample taken from the reboiler at the end ofrun indicates 0.2% ammonia, 14.8% methanol, 3.3% dimethyl carbonate,22.4% methyl carbamate, 58.4% triglyme and 0.9% N-MMC. The overheadproduct contained 9.0% dimethyl carbonate. The content of urea in thebottom product sample was unknown because urea could not be analyzed bygas chromatography due to urea decomposition.

EXAMPLE 5B

The experiment was carried out in the identical manner to the EXAMPLE 5Abut without the cocatalyst. The reboiler was charged with 125 g methylcarbamate, 200 g methanol and 24.6 g dibutyltin dimethoxide. Thereboiler temperature was maintained at 352-356° F. by controlling theoverhead pressure during the reaction. The flow rate of the overheadliquid product was set at 2.5 cc/min. A methyl carbamate solutionprepared by dissolving 125 g methyl carbamate in 2000 g methanol waspumped into the reboiler to maintain the constant liquid level in thereboiler. The reaction was terminated after 12 hours on stream. Theoverhead pressures at the beginning and the end were 207.7 and 299.5psig respectively. The column temperatures at the bottom and top sectionof the column were 317° F. and 305° F. at the beginning, and 342° F. and331° F. at the end of 12 hours run respectively. The analysis of thesample taken from the reboiler at the end of run indicated 0.4% ammonia,60.8% methanol, 6.7% dimethyl carbonate, 27.6% methyl carbamate, 4.2%N-MMC and 0.2% unknown. The overhead product contained 4.7% dimethylcarbonate and 0.1% methylamine. The result is listed in the Table 2. Thecontent of urea in the bottom product sample was unknown because ureacould not be analyzed by gas chromatography due to urea decomposition.

EXAMPLES 4A and 4B indicate slow thermal decomposition of methylcarbamate and that the organotin compounds can be effective catalystsfor the decomposition of methyl carbamate and urea in the absence ofmethanol. Too low a concentration of methanol in the reaction medium canpromote the decomposition of methyl carbamate or urea or both. Too highconcentration of methanol will slow down the reaction rate and causehigh reactor pressure which will cause difficulty in distilling offdimethyl carbonate from the reboiler resulting in higher by-productformation. Therefore, it is important to maintain an optimumconcentration of methanol in the reaction zone.

The flow rate of the overhead liquid product was increased to 2.5 cc/minin the EXAMPLES 5A and 5B. Methyl carbamate solutions were pumped intothe reboiler of the distillation still during the run to simulate thecontinuous operation. When the reaction was carried out as at the highflow rate there was little improvement compared with lower flow rate(1.5 cc/m in EXAMPLE 3A, Table 2). The increased overhead rate simplydiluted the concentration of dimethyl carbonate from 16-18% in theEXAMPLE 3A to about 9%, which is undesirable for the separation ofdimethyl carbonate from the overhead liquid product. When the reactionwas carried out in the absence of triglyme solvent (EXAMPLE 5B), theconcentration of dimethyl carbonate in the overhead product was nearlydoubled with improved apparent selectivity of dimethyl carbonate and theproduction of much less methylamine than in the EXAMPLE 3B, althoughlittle change in N-MMC formation was noticed. The concentration ofdimethyl carbonate in the overhead liquid product was only about half ofthe EXAMPLE 5A, resulting in about ⅓ less removal of dimethyl carbonateas the overhead liquid product compared with the EXAMPLE 5A as shown inTable 1. Although the concentration of urea in the bottom products isnot known, the conversion of methyl carbamate appears to be higher forthe EXAMPLE 5A than 5B due to effective removal of both dimethylcarbonate and ammonia from the reaction zone.

TABLE 2 Example 5A 5B Solvent in 100 MeOH-100 Triglyme 200 MeOH reboilerReboiler 355 355 Temp, ° F. Ovhd P, psig initial 39.4 207.7 final 123.9299.5 Ovhd Prod 2.5 2.5 Rate, cc/min Mass Balance, % 96.1 98.7 MoleBalance*, % 88.9 87.5 App. MC Conv*, % 82.9 67.6 App. Selectivity*, m %DMC 88.7 72.2 N-MMC 0.9 8.4 DMC recovered 169.7 108.3 as Ovhd Prod, g*The urea content in the reboiler was not included in the calculation.

EXAMPLE 6A

The reboiler of the distillation still was charged with 125 g methylcarbamate, 100 g methanol, 100 g triglyme and 24.8 g dibutyltindimethoxide. The reboiler temperature was maintained at 353-357° F. bycontrolling the overhead pressure during the reaction. The flow rate ofthe overhead liquid product was set at 2.7 cc/min. A urea solutionprepared by dissolving 105.6 g urea in 2200 g methanol was pumped intothe reboiler to maintain a constant liquid level in the reboiler. Thereaction was terminated after 12 hours uninterrupted continuousoperation. The result of this experiment is listed in Table 3. Thechange in the composition of dimethyl carbonate in the overhead productsduring the run is illustrated in FIG. 5. The overhead pressures at thebeginning and the end were 47.9 psig and 49 psig respectively. Thecolumn temperatures at the bottom and top section of the column were231° F. and 219° F. at the beginning and 232° F. and 200° F. at the endof 12 hours run respectively. While the analysis of the sample takenfrom the reboiler at the end of 12 hours run indicated 1.1% dimethylcarbonate, 13.1% methanol, 30.2% methyl carbamate, 0.8% N-MMC, 54.3%triglyme and 0.5% unknown, the overhead product contained 7.3% dimethylcarbonate and no methylamine. All the overhead samples taken during the12 hours run contained no detectable amount of methylamine. The contentof urea in the bottom product sample was unknown because urea could notbe analyzed by gas chromatography due to urea decomposition.

EXAMPLE 6B

The reaction was carried out by utilizing the distillation stilldisclosed herein as in Example 6A but without the cocatalyst. Thereboiler of the distillation still was charged with 125 g methylcarbamate, 200 g methanol, and 25.6 g dibutyltin dimethoxide. Thereboiler temperature was maintained at 352-356° F. by controlling theoverhead pressure during the reaction. The flow rate of the overheadliquid product was set at 2.7 cc/min. A urea solution prepared bydissolving 105.6 g urea in 2000 g methanol was pumped into the reboilerto maintain the constant liquid level in the reboiler. The reaction wasterminated after 12 hours uninterrupted continuous operation. The resultof this experiment is listed in Table 3. The change in the compositionof dimethyl carbonate in the overhead product during the run isillustrated in FIG. 5. The overhead pressures at the beginning and theend were 222 psig and 236.7 psig, respectively. The column temperaturesat the bottom and top section of the column were 332° F. and 311° F. atthe beginning and 326° F. and 314° F. at the end of 12 hours run. Whilethe analysis of the sample taken from the reboiler at the end of 12hours run indicates 8.2% dimethyl carbonate, 45.3% methanol, 37.3%methyl carbamate, 8.0% N-MMC, and 0.8% unknown, the overhead samplestaken during the first 6 hours run contained no detectable amount ofmethylamine. The overhead samples during the second 6 hours runcontained 0.08 to 0.1% methylamine. The content of urea in the bottomproduct sample was unknown because urea could not be analyzed by gaschromatography due to urea decomposition.

TABLE 3 Example 6A 6B Solvent 100 MeOh-100 Triglyme 200 MeOH Reboiler355 355 Temp, ° F. Ovhd P, psig Initial 47.9 222 Final 49 236.7 OvhdProd 2.7 2.7 Rate, cc/min Mass Balance, % 95.9 96.5 DMC produced (m %)*53.6 49.4 N-MMC produced (m %)* 0.5 5.8 Methylamine (%) in ovhd 0 0-0.1DMC recovered 153.5 129.3 as Ovhd Prod, g *Calculated based on thecombined total methyl carbamate and urea charged into the reboilerduring the run.

The EXAMPLE 6A simulates the continuous run for which an urea solutionis pumped into the reboiler (reactor) of the distillation still toconvert urea to dimethyl carbonate. The experiments demonstrate thaturea can be converted to dimethyl carbonate in one step, that is, it isnot necessary to convert urea to methyl carbamate in one reactor andthen convert methyl carbamate to dimethyl carbonate in another reactorbecause the extra amount of ammonia produced by the Reaction 1 caneffectively be removed from the reaction zone (reboiler), when urea isconverted to methyl carbamate although one may choose to producedimethyl carbonate in two steps. As shown in Table 3, when the reactionwas carried out according to the preferred embodiment, a superior resultwas obtained again; higher productivity of dimethyl carbonate and lowerformation of by-products. When triglyme was not used as cocatalyst aswell as solvent the amount of N-MMC produced is 10 times higher than thereaction carried out according to the present invention.

Since, to convert urea to methyl carbamate (Reaction 1), urea has toreact with methanol, that is, methyl carbamate has to compete with ureafor methanol to produce dimethyl carbonate (Reaction 2). The analysis ofbottom product taken after 12 hours run indicates that the methanolconcentration in the EXAMPLE 1A (with cocatalyst) appears to be too low(13.1%) for Reaction 1 and Reaction 2 to occur consecutively withoutcompetition for methanol. The amount of triglyme in the reboiler of thedistillation still was too high at the start of the run indicating thatwhen an urea solution is pumped into the reboiler to produce dimethylcarbonate in a single step the ratio of triglyme to methanol used toconvert methyl carbamate to dimethyl carbonate should be readjusted,e.g., lowered.

EXAMPLE 7

This example illustrates the actual production of DMC by one step.

The reboiler of the distillation still was charged with 125 g methylcarbamate, 120 g methanol, 80 g triglyme and 25 dibutylin dimethoxide.The reboiler temperature was maintained at 349-357° F. by controllingthe overhead pressure during the 12 hours uninterrupted run. The flowrate of the overhead liquid product was set at 2 cc/min. A urea solutionprepared by dissolving 105.6 g urea in 2200 g methanol was pumped intothe reboiler to maintain a constant liquid level in the reboiler. Thereaction was terminated after 12 hours uninterrupted operation. Theresult of this experiment is listed in Table 5 4. The change of the DMCcomposition in the overhead products is shown in FIG. 6. The overheadpressures at the beginning and the end of 12 hours run were 66 and 134.7psig, respectively. The column temperatures at the bottom and topsection of the column were 248° F. and 233° F. at the beginning, and286° F. and 274° F. at the end of 12 hours, respectively. While theanalysis of the sample taken from the reboiler a the end of 12 hours runindicated 3.8% dimethyl carbonate, 20.9% methanol, 21.1% methylcarbamate, 1.5% N-MMC, 52.0% triglyme, 0.2% unknown, 0.2% methylamine(or water) and 0.3% ammonia, the overhead product contained 9.0%dimethyl carbonate, 88.4% methanol, 0.1% methylamine (or water) and 2.5%ammonia. The content of urea in the bottom product sample was unknownbecause urea could not be analyzed by gc due to urea decomposition. Theunit was shut down for the next day's run. The weight of the compositeoverhead product was 1054 g and the weight of the urea solution pumpedinto the reboiler was 1252 g. The total samples taken out from the unitwas 210.8 g. There was lower liquid level in the reboiler from 8 to 12hours on stream. The composite overhead product contained 11.5% dimethylcarbonate. A vent gas was collected for 12 hours during the reaction(very little gas volume) and the analysis of this vent gas indicated0.05 vol % CO₂ and 2.1 vol O₂ indicating very little decomposition ofmethyl carbamate or urea.

The run was continued the next day by pumping a mixed solution preparedby mixing 1650 g methanol with 142.5 g triglyme into the reboiler. Thereboiler temperature was maintained at 348-359° F. by controlling theoverhead pressure. The flow rate of the overhead liquid product was seta 2 cc/min. The reaction was terminated after 10 hours uninterruptedoperation. The result of this experiment is listed in Table 4. Theoverhead pressures at the beginning and the end of 10 hoursuninterrupted run were 232.1 and 201.7 psig, respectively. The columntemperatures at the bottom and top section of the column were 248° F.and 233° F. at the beginning, and 322° F. and 313° F. at the end of 10hours run, respectively. While the analysis of the sample taken from thereboiler at the end of 10 hours (total 22 hours from the very beginning)run indicated 1.7% dimethyl carbonate, 22.2% methanol, 1.5% methylcarbamate, 1.3% N-MMC, 71.9% triglyme, 1.3% unknowns and 0.1% air, theoverhead product contained 3.8% dimethyl carbonate, 94.94% methanol and1.2% ammonia. The content of urea in the bottom product sample wasunknown, because urea could not be analyzed by gc due to ureadecomposition. The weight of the composite overhead product was 956 gand the weight of the mixed solution pumped into the reboiler was 1088.The total weight of the samples taken out from the unit was 197.2 g. Thetotal weight of the inventory material collected from the column and thereboiler was 249. The vent gas was collected during the run (very smallgas volume) and it contained 10.0 vol % CO₂ and 0.7 vol O₂. The changeof DMC composition in the overhead product for the second portion of therun is included in FIG. 6. High DMC concentration in the overheadproduct is very desirable because the DMC separation is a costly processdue to the formation of binary azeotrope with methanol. As shown inTable 4 the DMC selectivity is excellent (98.2%). When the urea solutionis pumped directly into the reboiler in the one step synthesis process,very little methyl carbamate decomposes resulting in the bestselectivity so far.

TABLE 4 Solvent 120 MeOH/80 TG Pump-in Solution Urea in MeOH(12 hrs)Meoh/TG(10 hrs) Reboiler Temp, ° F. 349-357 358-359 Ovhd P, psig Initial66 232.1 Final 134.7 201.7 Ovhd Product Rat, cc/m 2 2 Mass Balance, %100.1 Mole Balance*, % 99.8 App. Conversion*, % 98.3 Selectivity*, % DMC98.2 N-MMC 1.6 DMC Recovered as 151.3¹ 209.1² Ovhd Product, g App.;apparent *Calculated based on combined methyl carbamate and ureaconsumed during the reaction, assuming no uncovered urea left in thereactor at the end of run. ¹For the first 12 hours run. ²For the total22 hours run.

The invention claimed is:
 1. A process for the production of dialkylcarbonates comprising the steps of: (a) feeding urea and a primaryalcohol to a reaction zone; (b) feeding an organotin compound selectedfrom the group consisting of R′₂ SnX ₂ (X═Cl, R′O, R′COO or R′COS), R′ ₃SnX, R′ ₂ SnO, Ph _(3-n) R′SnX _(n) or Ph _(4-n) SnX _(n) (wherein R′=C_(q) H _(2q-1) , n=0, 1 or 2 and q=1 to 12 ) and mixtures thereof and ahigh boiling electron donor atom containing solvent comprising bidentateligands which form 1:1 bidentate and/or 1:2 monodentate comprisingmaterials having the general formula RO[CH₂(CH ₂)_(k) CH ₂ O] _(m) R,wherein each R is independently selected from C ₁₋₁₂ alkyl, alkaryl oraralkyl moieties, k=0, 1, 2 or 3 and m=1, 2, 3, 4, or 5 and mixturesthereof, to said reaction zone; and (c) concurrently in said reactionzone (i) forming an adduct of said organotin compound and saidbidentated ligands, (ii) reacting a portion of the primary alcohol andurea in the presence of said organotin compound and said high boilingelectron donor atom containing solvent to produce dialkyl carbonate; and(ii)(iii) removing the dialkyl carbonate and from said reaction zone asa vapor mixture; and (iv) recovering the dialkyl carbonate from saidvapor mixture by condensation.
 2. The process according to claim 1wherein ammonia and a portion of said alcohol are removed from saidreaction zone as vapor and withdrawn along with said dialkyl carbonateas overheads.
 3. The process according to claim 2 wherein said overheadsare partially condensed to separate said ammonia as a vapor from saiddialkyl carbonate and said alcohol as a liquid.
 4. The process accordingto claim 1 wherein said organotin catalyst is dibutyltin dimethoxide. 5.The process according to claim 1 wherein said high boiling electrondonor atom containing solvent comprises polyglycol ether.
 6. The processaccording to claim 1 wherein said high boiling electron donor atomcontaining solvent comprises (1) materials having the general formulaRO[CH₂(CH₂)_(k)CH₂O]_(m)R, wherein each R is independently selected fromC₁₋₁₂ alkyl, alkaryl or aralkyl moieties, k=0, 1, 2 or 3 and m=1, 2, 3,4 or 5 and (2) bidentate ligands which form 1:1 bidentate and/or 1:2monodentate adducts with R′₂SnX₂(X=Cl, R′O, R′COO or R′COS), R′₃SnX,R′SnO, Ph_(3-n)R′SnX_(n) or Ph_(4-n)SnX_(n) (wherein R′=C_(q)H_(2q-1)n=0, 1 or 2 and q=2 to 12) and mixtures thereof.
 7. The processaccording to claim 6 1wherein said high boiling electron donor atomcontaining solvent comprises triethylene glycol dimethyl ether.
 8. Theprocess according to claim 1 wherein said high boiling electron donoratom containing solvent comprises materials having the general formulaRO[CH₂(CH₂)_(k)CH₂O]_(m)R, wherein each R is independently selected fromC₁₋₁₂ alkyl, alkaryl or aralkyl moieties, k=0,1, 2 or 3 and m=1, 2, 3, 4or 5 and mixtures thereof.
 9. The process according to claim 1 whereinsaid high boiling electron donor atom containing solvent comprisesbidentate ligand which form 1:1 bidentate and/or 1:2 monodentate adductswith R′₂SnX₂(X=Cl, R′O, R′COO or R′COS), R′₃SnX, R′SnO,Ph_(3-n)R′SnX_(n) or Ph_(4-n)SnX_(n) (wherein R′=C_(q)H_(2q-1), n=0, 1or 2 and q=2 to 12) and mixtures thereof.
 10. The process according toclaim 1 wherein said primary alcohol is methanol and said dialkylcarbonate is dimethyl carbonate.
 11. A process for the production ofdimethyl carbonate comprising the steps of: (a) feeding urea andmethanol to the reboiler of a distillation still; (b) feeding dialkyltincatalyst and triethylene glycol dimethyl ether solvent/cocatalyst tosaid reboiler; (c) concurrently in said reboiler; (i) forming an adductof said organotin compound and said bidentated ligands, (ii) reacting aportion of said methanol and urea in the presence of said dibutyltincatalyst and said triethylene glycol dimethyl ether cocatalyst adduct tofinally produce dimethyl carbonate; and (ii)(iii) removing the dimethylcarbonate and ammonia from said reboiler as a vapor mixture; and (iv)recovering the dimethyl carbonate from said vapor mixture bycondensation.
 12. The process according to claim 11 wherein a portion ofsaid methanol is removed from said reboiler as vapor and withdrawn fromsaid distillation column along with said dimethyl carbonate asoverheads.
 13. The process according to claim 12 wherein said overheadsare partially condensed to separate said dimethyl carbonate and saidmethanol as a liquid.
 14. The process according to claim 1 wherein saidorganotin compound comprises R′₂ SnX ₂.
 15. The process according toclaim 1 wherein said organotin compound comprises R′₃ SnX.
 16. Theprocess according to claim 1 wherein said organotin compound comprisesR′₂ SnO.
 17. The process according to claim 1 wherein said organotincompound comprises Ph_(3-n) R′SnX _(n).
 18. The process according toclaim 1 wherein said organotin compound comprises Ph_(4-n) SnX _(n).